From c4a8cbd15a471d2a658de96abcc3254fc95de1bf Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 22 Mar 2016 16:06:21 +0000
Subject: [PATCH] Make LruCache use a dedicated _Node class

---
 synapse/util/caches/lrucache.py | 73 ++++++++++++++++++---------------
 1 file changed, 41 insertions(+), 32 deletions(-)

diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py
index f7423f2fab..f9df445a8d 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -29,6 +29,16 @@ def enumerate_leaves(node, depth):
                 yield m
 
 
+class _Node(object):
+    __slots__ = ["prev_node", "next_node", "key", "value"]
+
+    def __init__(self, prev_node, next_node, key, value):
+        self.prev_node = prev_node
+        self.next_node = next_node
+        self.key = key
+        self.value = value
+
+
 class LruCache(object):
     """
     Least-recently-used cache.
@@ -38,10 +48,9 @@ class LruCache(object):
     def __init__(self, max_size, keylen=1, cache_type=dict):
         cache = cache_type()
         self.cache = cache  # Used for introspection.
-        list_root = []
-        list_root[:] = [list_root, list_root, None, None]
-
-        PREV, NEXT, KEY, VALUE = 0, 1, 2, 3
+        list_root = _Node(None, None, None, None)
+        list_root.next_node = list_root
+        list_root.prev_node = list_root
 
         lock = threading.Lock()
 
@@ -55,36 +64,36 @@ class LruCache(object):
 
         def add_node(key, value):
             prev_node = list_root
-            next_node = prev_node[NEXT]
-            node = [prev_node, next_node, key, value]
-            prev_node[NEXT] = node
-            next_node[PREV] = node
+            next_node = prev_node.next_node
+            node = _Node(prev_node, next_node, key, value)
+            prev_node.next_node = node
+            next_node.prev_node = node
             cache[key] = node
 
         def move_node_to_front(node):
-            prev_node = node[PREV]
-            next_node = node[NEXT]
-            prev_node[NEXT] = next_node
-            next_node[PREV] = prev_node
+            prev_node = node.prev_node
+            next_node = node.next_node
+            prev_node.next_node = next_node
+            next_node.prev_node = prev_node
             prev_node = list_root
-            next_node = prev_node[NEXT]
-            node[PREV] = prev_node
-            node[NEXT] = next_node
-            prev_node[NEXT] = node
-            next_node[PREV] = node
+            next_node = prev_node.next_node
+            node.prev_node = prev_node
+            node.next_node = next_node
+            prev_node.next_node = node
+            next_node.prev_node = node
 
         def delete_node(node):
-            prev_node = node[PREV]
-            next_node = node[NEXT]
-            prev_node[NEXT] = next_node
-            next_node[PREV] = prev_node
+            prev_node = node.prev_node
+            next_node = node.next_node
+            prev_node.next_node = next_node
+            next_node.prev_node = prev_node
 
         @synchronized
         def cache_get(key, default=None):
             node = cache.get(key, None)
             if node is not None:
                 move_node_to_front(node)
-                return node[VALUE]
+                return node.value
             else:
                 return default
 
@@ -93,25 +102,25 @@ class LruCache(object):
             node = cache.get(key, None)
             if node is not None:
                 move_node_to_front(node)
-                node[VALUE] = value
+                node.value = value
             else:
                 add_node(key, value)
                 if len(cache) > max_size:
-                    todelete = list_root[PREV]
+                    todelete = list_root.prev_node
                     delete_node(todelete)
-                    cache.pop(todelete[KEY], None)
+                    cache.pop(todelete.key, None)
 
         @synchronized
         def cache_set_default(key, value):
             node = cache.get(key, None)
             if node is not None:
-                return node[VALUE]
+                return node.value
             else:
                 add_node(key, value)
                 if len(cache) > max_size:
-                    todelete = list_root[PREV]
+                    todelete = list_root.prev_node
                     delete_node(todelete)
-                    cache.pop(todelete[KEY], None)
+                    cache.pop(todelete.key, None)
                 return value
 
         @synchronized
@@ -119,8 +128,8 @@ class LruCache(object):
             node = cache.get(key, None)
             if node:
                 delete_node(node)
-                cache.pop(node[KEY], None)
-                return node[VALUE]
+                cache.pop(node.key, None)
+                return node.value
             else:
                 return default
 
@@ -137,8 +146,8 @@ class LruCache(object):
 
         @synchronized
         def cache_clear():
-            list_root[NEXT] = list_root
-            list_root[PREV] = list_root
+            list_root.next_node = list_root
+            list_root.prev_node = list_root
             cache.clear()
 
         @synchronized